The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis about which it revolves in normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
A circumferential plane is a plane perpendicular to the axis of rotation of the tire.
The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which splits the tire into two halves.
Such vehicles, generally intended to carry heavy loads, need to meet certain requirements regarding in particular the longitudinal spacing between each of the axles. Specifically, the various sets of regulations dictate longitudinal distances between two axles of a trailer that has three axles, according to the permissible load for said vehicle. This is usually a distance of 1.3 meters for trailers comprising three axles. Moreover, current market trends for these trailers is towards a standardization of the design of these trailers and particularly towards equipping them with a given and the same tire for all of the axles.
The way in which such three-axle trailers are used has an effect notably on tire wear. Specifically, when the vehicle follows a straight-line path, for example on a road or a motorway, all of the tires with which the trailer is fitted experience substantially equivalent conditions. However, as soon as the trailer follows a curved path, such as on a roundabout for example or during maneuvering, the tires are no longer loaded in the same way. These differences in load generate highly variable levels of wear on the various tires with which the trailer is fitted, making tire management in particular complicated. It also entails the provision of tires that can meet the most severe tire wear conditions and therefore tires that have a sufficient depth of tread. This of course makes the design of the tire more complicated, because the mass of rubber is not favorable to tire durability because it leads to increases in tire temperature during running.
First of all, notably as a function of speed, running along a curved path leads to load being transferred onto the tires that are on the outside of the bend, and therefore to greater wear thereof.
Secondly, some of the tires fitted to the three axles experience phenomena of scrubbing against the ground, which accentuates the wearing of the tires affected by these phenomena. Indeed it is known that when the vehicle is fitted with three axles, each fitted with at least two tires, not all of the tires are able to follow a curved path corresponding to the path followed by the vehicle. When the spacing between the axles is even, if the tires fitted to the intermediate axle follow a path substantially equivalent to that of the vehicle, the tires fitted to the other two axles experience phenomena of scrubbing against the ground, leading to greater tire wear.
It is clear from this usage that the tire wear of tires fitted to such a vehicle is uneven, and dependent on whereabouts on the trailer the tire is fitted, even though the tires are usually all the same.
Further, changes to road layouts, notably with a view to improving the safety of road users, has led to a proliferation of roundabouts and therefore to a not-insignificant increase in the causes that lead to differential tire wear across the tires with which a trailer is fitted.
The current state of affairs means that uneven tire wear on a trailer comprising three axles is becoming troublesome to the user who too often has to lay up his vehicle to remedy this, either by swapping the tires around or by changing them.
Document FR 2 903 953 or even document EP 1 640 247 propose solutions consisting in self-steering axles which work by passive or even active steering of the axles of a trailer. While these technologies provide solutions to the scrubbing problem or to the problem of differential tire wear, they are, on the other hand, difficult and expensive to implement. Further, because of their complexities, these technologies are operational only during maneuvering.
There are also available on the market such trailers in which one or more axles can be raised up to eliminate contact between the tires with which these axles are fitted and the ground. These raisable axles of course avoid tire wear for the tires concerned when they are in the raised position, but this status can be maintained only as long as the load being carried is below the maximum possible load.